Chip resistor

ABSTRACT

A chip resistor reveals a crack for permitting easy detection of it in the inspection process, suffers from minimum variation of the resistance during calcination of a protection film, and is not prone to defects such as pinholes that do not come to the surface. This chip resistor is produced by forming a resistive layer on the surface of an insulating substrate, providing electrodes at both ends of the resistive layer, forming a resistive-layer protection film on the surface of the resistive layer, forming an intermediate protection film on the surface of the resistive-layer protection film, and forming a surface protection film on the surface of the intermediate protection film. In addition, in this chip resistor, the resistive-layer protection film, intermediate protection film, and surface protection film are all made of an identical material.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to an improvement on a chipresistor.

[0003] 2. Description of the Prior Art

[0004] Chip resistors that have conventionally been in wide use areproduced by forming a resistive layer on the s e of an insulatingsubstrate, providing electrodes both ends of the resistive layer, andforming one or more protective films on the surface of the resistivelayer. FIG. 4 shows a vertical section of such a chip resistor. Thisfigure shows a chip resistor having three protective films formed on thesurface of its resistive layer, with numeral 1 representing aninsulating substrate made of, for example, ceramics, numeral 2representing a resistive layer formed on the surface of the substrate 1,numeral 3 representing electrodes provided at both ends of the resistivelayer 2, numeral 4 representing a resistive-layer protection film,numeral 5′ representing an intermediate protection film, and numeral 6′representing a surface protection film. Each protection film is madeessentially of glass paste. The electrodes 3 have their surfacesmetal-plated.

[0005] The protection films are formed as follows. First, the materialfor the resistive-layer protection film 4 is applied to the surface ofthe resistive layer 2, and is then subjected to drying and calcination.At this time, the resistive-layer protection film 4 serves to reduce thevariation of (i.e. stabilize) the resistance of the resistive layer 2under calcination. Thereafter, the resistive layer 2 is trimmed, forexample, with a laser beam for the adjustment of its resistance.Subsequently, the intermediate protection film 5′ is applied to thesurface of the resistive-layer protection film 4 and is then subjectedto drying. Subsequently, the surface protection film 6′ is applied tothe surface of the intermediate protection film 5′ and is then subjectedto drying. Lastly, the surface protection film 6′ is subjected tocalcination. It is also possible to subject the intermediate protectionfilm 5′ to calcination before the application, drying, and calcinationof the surface protection film 6′.

[0006] In general, the resistive-layer protection film 4 is provided, asdescribed above, for the purpose of reducing the variation of theresistance of the resistive layer 2 under calcination; the intermediateprotection film 5′ is provided for the purpose of filling trimminggrooves that are left after the above-mentioned trimming; the surfaceprotection film 6′ is provided for the purpose of protecting theresistor against mechanical force that may be applied from outside.Thus, in a conventional chip resistor, these protection films, to servetheir respective intended purposes, need to be made of materials havingdifferent properties in terms of their softening point, Vickershardness, thermal expansion coefficient, and others. This leads to thefollowing inconveniences.

[0007] For one thing, when a chip resistor, in the manufacturingprocess, receives mechanical force from outside, the chip resistor maydevelop, as shown in FIG. 5, a crack that penetrates completely throughthe surface protection film 6′ and the intermediate protection film 5′but only halfway into the resistive-layer protection film 4. In actualuse, a chip resistor with such a crack, when heat is applied theretoduring soldering, often ends in the crack reaching the resistive layer 2and thus the chip resistor having a resistance different from theintended resistance. In this case, exactly because each protection filmis made of a different material, the crack tends to take a non-linearpath and thus remain inside, without coming to the surface.

[0008] Alternatively, in cases where the intermediate protection film 5′is made of a mechanically weak material, the chip resistor may develop,as shown in FIG. 6, multiple cracks in the intermediate protection film5′, and in addition the differences in the thermal expansion coefficientbetween the protection films cause stress to be present at all timesbetween those films. This makes the chip resistor susceptible to athermal shock such as is caused by soldering.

[0009] Moreover, the difference in the softening point between theintermediate protection film 5′ and the surface protection film 6′ makesit difficult to determine the appropriate calcination temperature. Thisleads to instability of the resistance of the resistive layer 2 undercalcination, or causes, in the protection films, defects such aspinholes that do not come to the surface. These faults are difficult todetect in the inspection process, and thus chip resistors having such afault are in many cases shipped out as non-defective products, withtheir fault unnoticed.

SUMMARY OF THE INVENTION

[0010] An object of the present invention is to provide a chip resistorthat reveals a crack for permitting easy detection of it in theinspection process, that suffers from minimum variation of theresistance during calcination of a protection film, and that is notprone to defects such as pinholes that do not come to the surface.

[0011] To achieve the above object, according to the present invention,in a chip resistor produced by forming a resistive layer on the surfaceof an insulating substrate, providing electrodes at both ends of theresistive layer, forming a resistive-layer protection film on thesurface of the resistive layer, forming an intermediate protection filmon the surface of the resistive-layer protection film, and forming asurface protection film on the surface of the intermediate protectionfilm, the resistive-layer protection film, the intermediate protectionfilm, and the surface protection film are all made of an identicalmaterial.

[0012] Alternatively, in a chip resistor produced by forming a resistivelayer on the surface of an insulating substrate, providing electrodes atboth ends of the resistive layer, forming a resistive-layer protectionfilm on the surface of the resistive layer, and forming a surfaceprotection film on the surface of the resistive-layer protection film,the resistive-layer protection film and the surface protection film areboth made of an identical material.

[0013] In these chip resistors, the protection films are madeessentially of lead-borosilicate glass of an identical composition. Morespecifically, the lead-borosilicate glass preferably has the followingproperties: Softening point: 570-620° C.; Vickers hardness: 400-600 Hv(after submission to a load of 200 g for 30 s); and Thermal expansioncoefficient: 40-70 × 10⁻⁷/° C. (in a temperature range of 30-300° C. ).

[0014] Moreover, the protection films are preferably made of glass pastethat contains lead-borosilicate glass in the form of particles 2-10 μmacross and that contains terpineol or butyl carbitol acetate as solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] This and other objects and features of this invention will becomeclear from the following description, taken in conjunction with thepreferred embodiments with reference to the accompanied drawings inwhich:

[0016]FIG. 1 is a vertical section illustrating the structure of a chipresistor embodying the invention;

[0017]FIG. 2 is a diagram showing an example of a crack that may occurin the chip resistor of the invention;

[0018]FIG. 3 is a diagram showing another example of a crack that mayoccur in the chip resistor of the invention;

[0019]FIG. 4 is a vertical section illustrating the structure of aconventional chip resistor;

[0020]FIG. 5 is a diagram showing an example of a crack that may occurin the conventional chip resistor; and

[0021]FIG. 6 is a diagram showing another example of a crack that mayoccur in the conventional chip resistor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] Hereinafter, an embodiment of the present invention will bedescribed with reference to the drawings. FIG. 1 shows the structure ofa chip resistor 10 embodying the invention. Numeral 1 represents aninsulating substrate made of, for example, ceramics, numeral 2represents a resistive layer formed on the surface of the substrate 1,and numeral 3 represents electrodes provided at both ends of theresistive layer 2. The components so far mentioned are the same as thecorresponding components in the conventional chip resistor 20 shown inFIG. 4. Numeral 4 represents a resistive-layer protection film, numeral5 represents an intermediate protection film, and numeral 6 represents asurface protection film.

[0023] To overcome the inconveniences mentioned earlier, in the chipresistor 10 of the invention, the protection films 4 to 6 are all madeof an identical material. As a result, when force is applied fromoutside vertically to the protection films of the chip resistor asindicated by arrow A in FIG. 2, the chip resistor develops a crack thatreaches the resistive layer 2 and thus makes the resistance differentfrom the intended resistance. This makes it possible, in the inspectionprocess, to detect the crack by measuring the resistance and therebyreject chip resistors having such a crack as being defective.Alternatively, when force is applied from outside at an angle to theprotection films of the chip resistor as indicated by arrow B in FIG. 3,the chip resistor develops a crack that comes back to the surface andthus is recognizable as a crack. This makes it possible, in theinspection process, to detect the crack by use of an appearancerecognition system and thereby reject chip resistors having such a crackas being defective.

[0024] Moreover, since the surface protection film 6 and theintermediate protection film 5 have the same thermal expansioncoefficient as the resistive-layer protection film 4, calcination of thesurface protection film 6 and the intermediate protection film 5 causesminimum variation of the resistance. Furthermore, since the surfaceprotection film 6 and the intermediate protection film 5, when subjectedto calcination simultaneously, start to soften approximately at the sametime. This makes it possible to determine calcination conditions thatsuit both of these films and thereby minimize formation of defects suchas pinholes.

[0025] In the chip resistor 10 of the invention, the intermediateprotection film 5 and the surface protection film 6 are made of thematerial that has conventionally been used to make the resistive-layerprotection film 4. More specifically, these films are made essentiallyof glass that is prepared in the form of glass paste for easyapplication, with the glass and the glass paste having the followingproperties: (1) Properties of the Glass a) Chief Ingredient:Lead-Borosilicate Glass b) Softening Point: 570-620° C. c) VickersHardness: 400-600 Hv (after submission to a load of 200 g for 30 s) d)Thermal Expansion Coefficient: 40-70 × 10⁻⁷/° C. (in a temperature rangeof 30-300° C.).

[0026] (2) Properties of the Glass Paste 1) Glass Particle Diameter:2-10 μm 2) Solvent: Terpineol or Butyl Carbitol Acetate

[0027] (3) Film Thicknesses after Calcination 1) When the surfaceprotection film 6 is pigmented, Resistive-Layer Protection Film 4: 2-10μm (≧ the glass particle diameter) Intermediate Protection Film 5: 2-10μm (≧ the glass particle diameter) Surface Protection Film 6: 5-20 μm 2)When the intermediate protection film 5 is pigmented, Resistive-LayerProtection Film 4: 2-10 μm (≧ the glass particle diameter) IntermediateProtection Film 5: 5-20 μm Surface Protection Film 6: 2-10 μm (≧ theglass particle diameter)

[0028] Note that the thermal expansion coefficient above is close tothat of ceramics. Note also that, to fill the trimming groovesefficiently, it is possible, if necessary, to use for the intermediateprotection film 5 such glass paste that contains glass particles of acomparatively small diameter or that contains a comparatively largeproportion of solvent. In general, however, a glass particle diameter of6 to 8 μm is most preferable to achieve proper filling of the trimminggrooves and at the same time secure an adequate film thickness.

[0029] When the nominal resistance or other information is printed onthe surface of the chip resistor, pigment of black or other color isadded to the intermediate protection film 5 and the surface protectionfilm 6 to obtain sufficient contrast between the printed characters orother and the background. The other protection film may be lefttransparent, with or without color, but, when printing is applied, it ispreferable that it be colored. Note however that there are also somecases in which no pigment is required at all.

[0030] Although not shown in the figures, in some cases, only twoprotection films are required. For example, when the chip resistor isconveyed by use of a vacuum-absorption conveyor in the mounting processon a printed circuit board, its surface is required to be as flat aspossible; in such cases, forming only two protection films tends toresult in a better flatness than forming three protection films. Thereare also cases where trimming is performed before any protection film isformed on the surface of the resistive layer 2; in such cases, too, itsuffices to form only two protection films. In any case, what structureto adopt is determined in consideration of the desired mechanicalstrength and the production cost.

[0031] As described heretofore, the chip resistor according to thepresent invention provides the following advantages. It permits easydetection of a crack in the inspection process since the crack readilycomes to the surface, and thus it provides a resistance less affected bysoldering or the like. It allows all of its protection films to have anidentical thermal expansion coefficient, and thus it suffers fromminimum variation of the resistance during calcination of a protectionfilm. It allows all of its protection films to have an identicalsoftening point, and thus it is not prone to defects such as pinholesthat do not come to the surface.

What is claimed is:
 1. A chip resistor produced by forming a resistivelayer on a surface of an insulating substrate, providing electrodes atboth ends of said resistive layer, forming a resistive-layer protectionfilm on a surface of said resistive layer, forming an intermediateprotection film on a surface of said resistive-layer protection film,and forming a surface protection film on a surface of said intermediateprotection film, wherein said resistive-layer protection film, saidintermediate protection film, and said surface protection film are allmade of an identical material.
 2. A chip resistor as claimed in claim 1,wherein said resistive-layer protection film, said intermediateprotection film, and said surface protection film are made essentiallyof lead-borosilicate glass of an identical composition.
 3. A chipresistor as claimed in claim 2, wherein said lead-borosilicate glass hasa softening point of 570-620° C., a Vickers hardness of 400-600 Hv aftersubmission to a load of 200 g for 30 s, and a thermal expansioncoefficient of 40-70×10⁻⁷/° C. in a temperature range of 30-300° C.
 4. Achip resistor as claimed in claim 3, wherein said resistive-layerprotection film, said intermediate protection film, and said surfaceprotection film are made of glass paste that contains lead-borosilicateglass in a form of particles 2-10 μm across and that contains terpineolor butyl carbitol acetate as solvent.
 5. A chip resistor produced byforming a resistive layer on a surface of an insulating substrate,providing electrodes at both ends of said resistive layer, forming aresistive-layer protection film on a surface of said resistive layer,and forming a surface protection film on a surface of saidresistive-layer protection film, wherein said resistive-layer protectionfilm and said surface protection film are both made of an identicalmaterial.
 6. A chip resistor as claimed in claim 5, wherein saidresistive-layer protection film and said surface protection film aremade essentially of lead-borosilicate glass of an identical composition.7. A chip resistor as claimed in claim 6, wherein said lead-borosilicateglass has a softening point of 570-620 ° C., a Vickers hardness of400-600 Hv after submission to a load of 200 g for 30 s, and a thermalexpansion coefficient of 40-70×10⁻⁻⁷/° C. in a temperature range of30-300° C.
 8. A chip resistor as claimed in claim 7, wherein saidresistive-layer protection film and said surface protection film aremade of glass paste that contains lead-borosilicate glass in a form ofparticles 2-10 μm across and that contains terpineol or butyl carbitolacetate as solvent.